1. Field of the Invention
The present invention relates to surface mounting type piezoelectric components, such as a ceramic oscillator and a ceramic filter.
2. Description of the Related Art
Conventionally, in a surface mounting type piezoelectric component, in order to perform a cleaning process at the time of mounting the piezoelectric component on a substrate, a fluid-resistant characteristic is required for a product incorporating the piezoelectric component and the substrate.
In order to reliably provide a fluid-resistant characteristic, an adhesive agent is disposed between a cap and an insulating substrate upon which the piezoelectric component is mounted. More specifically, after mounting the piezoelectric component such as an oscillator element on the insulating substrate made of alumina, for example, and on which an electrode is disposed, a cap made from alumina is put on and sealed with the insulating substrate via the adhesive agent.
However, the cap made from alumina has very high molding and baking costs. Furthermore, a thickness of the cap must be at least about 0.5 mm because of the manufacturing process used to form the cap. Because the piezoelectric oscillator is usually used together with an integrated circuit (IC), it is preferable that the height or vertical dimension of a complete product incorporating the oscillator and the IC is equal to or less than that of the IC. The height or vertical dimension of the IC has become very small in recent years (for example, about 1.2 mm). For this reason, when using the cap made from alumina, sometimes the desired height of the product could not be realized.
In order to decrease the height or vertical dimension of the product, a metal cap has been proposed. Unexamined Japanese Patent Publication No. 8-111626 discloses a piezoelectric component, wherein a metal cap is fixed to a substrate and a space defined between the metal cap and the substrate is sealed via an adhesive agent. The adhesive agent has a physical property which alleviates the effect of any difference in a thermal expansion coefficient of the metal cap and of the substrate. In this case, the height or vertical dimension of a resulting product can be made low, and the piezoelectric element does not suffer negative effects such as a dipole and degradation of the sealing condition caused by heat shock or other environmental conditions can be suppressed.
Incidentally, a surface mounting type piezoelectric component is soldered to a circuit board via a reflow process. In the case of reflow soldering, a maximum temperature reaches 250 degrees centigrade, for example. Therefore, if the heat resistance of an adhesive agent is low, a sealing portion of an insulating substrate (circuit board) and a cap cannot tolerate an increase in the internal pressure of the cap accompanied by a temperature rise, and consequently, the cap is detached from the substrate.
Moreover, when the difference between the thermal expansion coefficients of the cap and the substrate is too large, even if the adhesive agent which has the physical property which alleviates the difference between thermal expansion coefficients is used, the cap detachment can not be prevented in many cases.